Let spatial shape tool block resizing attempt

[pllim]

This is a follow-up of this rejected PR:

• BUG: Prevent some spatial ROI from getting too small #416

Desired outcome:

• Let us tell the tool when it is okay to resize and when it is not to prevent spatial subset from disappearing (i.e., becomes very small suddenly) when user drags to move it around in Jdaviz.
• Or better yet, prevent wonky selected_x and selected_y inputs in the first place, though no one seems to know how to fix that.

🐱

[dhomeier]

Since I cannot reproduce the phenomenon on any of my machines, feel free to try the modification in #419